TMRR-01R Two Triarii Standing, Roman Army of the Mid Republic
For set-piece battles, the heavy infantry were usually drawn up in three lines.
However, the vast majority of the heavy infantry were stationed in the front two lines, the HASTATI and PRINCIPES. Contained in these two lines were the younger recruits who were expected to do all the fighting. The rear line (TRIARII), was a reserve consisting of older men who formed a line of last resort to provide cover for the front lines if they were put to flight (and also to prevent unauthorised retreat by the front ranks).
The three lines of maniples were drawn up in a chessboard pattern (dubbed quincunx by modern historians, after the Latin for the “5” on a dice-cube, whose dots are so arranged). In front of these three lines of heavy infantry, would be stationed the legion’s VELITES.
The Roman army of the mid-Republic (also known as the manipular Roman army or the “Polybian army”), refers to the armed forces deployed by the mid-Roman Republic, from the end of the Samnite Wars (290 BC) to the end of the Social War (88 BC). The first phase of this army, in its manipular structure (290–ca. 130 BC), is described in detail in the Histories of the ancient Greek historian Polybius, writing before 146 BC.
The central feature of the mid-Republican army was the manipular organisation of its battle-line. Instead of a single, large mass (the phalanx) as in the Greek and Early Roman army, the Romans now drew up in three lines (triplex acies) consisting of small units (maniples) of 120 men, arrayed in chessboard fashion, giving much greater tactical strength and flexibility.